ANAT 322 Lecture Notes - Lecture 3: Cerebral Cortex, Endocrinology, Portal Venous System
7 Jun 2018
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ANAT 322 Winter 2017
Lectures
Lecture 3:
→ Control of the Anterior Pituitary Lobe
3. This control is exerted in a true endocrine matter so there are no direct projections from the
parvocellular neurons to the anterior lobe of the pituitary gland. The axonal branches they extend to the
medial eminence terminate there and there are no direct projections into the anterior lobe.
4. High efficiency and high speed control of the hypothalamus over the anterior pituitary is achieved
through the portal blood supply system or hypophyseal blood supply system.
All the arterial blood supplied eventually to the anterior pituitary gland stems from a branch of the
superior hypopheseal artery (SHA). That artery is peculiar in that it capillarizes so it branches into small
capillaries that invade the tissue and supply oxygen there but also communicate with the extracellular
fluid, not at the end of the target of the pituitary gland, but at the median eminence at the base of the
pituitary stalk. There we get little capillaries formed from the SHA and little venules that coil around
capillaries and reform to a set of veins referred to as the hypopheseal portal veins.
The hypopheseal portal veins is a system of blood supply called portal as the one found in the liver in
which the mesenteric artery capillarizes in the epithelium of the gut take the nutrients derived from
food and supply that venous blood to the liver. Portal supply means that you get two organs that are in
series in the sequence of that blood supply. In our example, the second organ in line in here would be
the anterior lobe of the pituitary gland that does not really receive any technically arterial blood but all
the blood that goes there is venous. By definition blood becomes venous after it goes through a capillary
bed and it is collected in little venules so all the supply of oxygen and nutrients to the anterior pituitary
gland is venous. The advantage is that the hypophysiotropic hormones such as the release and the
release-inhibiting hormones from the hypothalamus are released in the medial eminence next to these
capillaries are still at very high concentrations when they get supplied to the pituitary gland which is the
target organ. The effect will not be as large and rapid if that were to go to the general blood supply and
arrive at the pituitary by that means so from a functional point of view this makes sense. These portal
veins form another bed of capillaries in the parenchyma of the anterior lobe where the secretion of the
hypophyseal hormones is happening and all of that blood gets collected in the hypophyseal veins that
drain in the cavernous sinus (CS) at the base of the brain.
The blood supply of the posterior lobe is simpler and straight forward because it is classical blood
supply for any endocrine glands. The arterial branch that supplies the posterior lobe is the inferior
hypophyseal artery (IHA) which establishes its capillary bed in the posterior lobe which then reform
venules and vein that also will drain through the hypophyseal veins into the cavernous sinus.
The way this portal system is design is important for the understanding of the function of the anterior
pituitary gland and how the hypothalamus exerts its effect on neuroendocrine cells in the pituitary.
5. The median eminence (ME) is a small region at the base of the hypothalamus and sometimes in
humans it can be at the frontal site of the pituitary stalk (the infundibulum). It is composed of two layers
and capillarization of SHA branches and the release of the release and the release-inhibiting hormones
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ANAT 322 Winter 2017
Lectures
happens in the external layer next to the surface of the brain or the meninges. In that region, the body
wants these macromolecules or peptide hormones to cross to the bloodstream to be taken into the
anterior pituitary and this is nervous system so a priori there is no open exchange of macromolecules
between extracellular space and the bloodstream because of the blood brain barrier. But some regions
like this one have an open blood brain barrier and these are called circumventricular organs which are
seven regions in the brain arranged around the ventricular system.
6. The median eminence is one of these and the posterior lobe of the pituitary gland can be added to
that because it is also nervous tissue and it is a site of release of neurohormones into the bloodstream.
Pineal gland is where hormone melatonin is being released into the bloodstream.
OVLT is near the bottom of the third ventricle near the optic chiasm and these are places where
neurons get a change to sense the chemical composition of blood plasma. OVLT and SFO are located
more anteriorly in the brain and there are neurons sensitive to peripheral hormones such as growth
hormone, insulin growth factors and can project to other regions in the hypothalamus and feedback
information.
Area postrema (AP) in the fourth ventricle becomes active if we have poisoning and senses the
presence of toxins and activates the throw out effect and we have no control over that.
7. Schematic drawing of a capillary and tissue in a place where the blood brain barrier is intact. All of the
endothelial cells have established tight and overlapping contact with one another and these contact
zones are closed with multiple tight junctions so there is no plasma exuding from vessels there under
normal healthy conditions and there is no extracellular fluids entering into the bloodstream.
The little dots are sites of pinocytosis so any transport of substances from plasma into tissue or vice
versa happens in an active manner and under the metabolic control of the cell and only the things that
the cell wants to transport can pass (transcytosis) or substances that are lipophilic that can just diffuse
through plasma membrane such as steroid hormones. Big macromolecules and things that are polar
cannot pass.
8. Fenestrated capillaries in the glomeruli in the nephrons of the kidney we also find fenestrated
capillaries which is a hallmark of all endocrine glands as well. Within each individual endothelial cell in
this capillary the cytoplasm forms more of a web rather than a continuous sheath of cytoplasm and
there are places where the barrier between blood plasma and the basal lamina of the endothelial cell
and then the extracellular space is only that one layer of plasma membrane and even the plasma
membrane forms little windows.
The only barrier between the contents of the blood vessel and the extracellular fluid is the basal
lamina and there is no cellular interference and these are the places where, following a gradient of
concentration from higher to lower concentration, releasing hormones are released into the
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Lectures
pericapillary space or extracellular space and the peptides can diffuse into blood plasma to be
transported to the anterior pituitary gland from there.
9. The internal layer at the base of the third ventricle that contains many axons or fibers from the
magnocellular neurons that travel to the posterior lobe of the pituitary gland.
Colocalization of neurons in the hypothalamus or brain and almost all of these neurons express co-
hormones so that GNRH for example can be expressed along with other hormones or neurotransmitters
that are also released in the median eminence. In addition there are vascular eminence important in the
median eminence as well as glial cells. A neurohemal organ is a technical term for a place in nervous
tissue in the CNS where neurohormones are released such as the median eminence.
10. Small enlargement from a histological slide of the base of the brain of a laboratory rat and the
animal was stained for CRH. With all the parvocellular hormones, the place in the brain where we find
the highest concentration of it is the median eminence and not even the cell bodies. The wall of the
third ventricle goes in a circle on top and bellow we are following then outer surface of the brain bellow
so the thickness is the median eminence. The external zone (2/3) is what is stained darker and the
internal zone (1/3) on top less stained in which the axons travel to the posterior lobe.
11. Type of glia that we find there is not found anywhere else in the brain and are referred to as
Tanycytes that have a fibrous appearance when stained. The cells bodies all sit in the zone under the
third ventricle and extend long processes to the internal zone and then they branch at the external zone
where we find the terminals of the parvocellular system and it is found in the developing nervous
system in vertebrates.
In mammalian adult brain, we only find this in the median eminence and the more general term for
them is radial glial because they form a radius form paraventricular zone to the surface of the brain. In
the developing brain, this type of glia serves as a scaffold along which differentiating neuroblasts
migrate to their final location or the maturing nervous system.
→ Regulatory Mechanisms in the Parvocellular System
13. Which levels of feedback can be found in a system where the hypothalamus regulates a master
endocrine gland, the pituitary, that regulates other endocrine glands in the body and there are three
levels of feedback.
Neurons in the body sense changes in hormone concentrations and report to the nervous system
and this sensing is the role of the circumventricular organs.
Ultrashort feedback occurs sometimes within the hypothalamus itself because there are also at least
one release-inhibiting hormone such as somatostatin where these neurons establish feedback loops
with neurons that make releasing hormones in the hypothalamus.
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Document Summary
Control of the anterior pituitary lobe: this control is exerted in a true endocrine matter so there are no direct projections from the parvocellular neurons to the anterior lobe of the pituitary gland. All the arterial blood supplied eventually to the anterior pituitary gland stems from a branch of the superior hypopheseal artery (sha). There we get little capillaries formed from the sha and little venules that coil around capillaries and reform to a set of veins referred to as the hypopheseal portal veins. Portal supply means that you get two organs that are in series in the sequence of that blood supply. In our example, the second organ in line in here would be the anterior lobe of the pituitary gland that does not really receive any technically arterial blood but all the blood that goes there is venous.
